Some of the goals of high speed packet access (HSPA) evolution include higher data rates, higher system capacity and coverage, enhanced support for packet services, reduced latency, reduced operator costs and backward compatibility. Meeting these goals requires evolutions to the radio interface protocol and network architecture. More specifically, meeting these goals has required a set of enhancements and architecture changes to layer 2 radio link control (RLC) and medium access control (MAC) functionalities.
In universal terrestrial radio access (UTRA) Release 6, an acknowledged mode (AM) RLC entity performs segmentation and concatenation of RLC service data units (SDUs) into fixed-size RLC packet data units (PDUs). The RLC PDU size is semi-static and can only be changed via higher layer signaling. The AM RLC entity is always re-established if the AM PDU size is changed by the higher layer. However, in the evolved HSPA architecture, in order to support high data rates, it has been proposed to have a flexible RLC PDU size that varies to reflect channel conditions. A flexible RLC PDU size increases RLC transmission and retransmission efficiency.
Flexible RLC PDU size will allow the radio network controller (RNC) to create RLC PDUs that closely reflect the channel conditions. A one-to-one mapping between an RLC PDU and a MAC PDU achieves the highest transmission efficiency. A bigger RLC PDU size may have a detrimental effect on the RLC transmission efficiency in bad channel conditions. Having a flexible PDU size would eliminate this problem since for the first RLC PDU transmission the size will reflect the current specified channel conditions, (i.e., max RLC PDU size).
However, when the RLC PDU size of the retransmission is larger than the current specified maximum RLC PDU size, a problem may occur for retransmission of the RLC PDU. If the channels conditions change dramatically the large RLC PDU may never be successfully transmitted. In addition, when a handover from Release 7 to Release 6 occurs, RLC PDUs created in Release 7 RLC might be larger than the new fixed RLC PDU size.